1. Field of the Invention
The present invention relates to an optical type rotary encoder for detecting a rotational angle of a rotatable body.
2. Description of the Related Art
In the field of industrial machinery, an optical type rotary encoder has been used as a sensor for detecting a rotational angle of a drive shaft of an electric motor or a rotatable body which is driven by the electric motor. In general, optical type rotary encoders are classified into transmission types which use modulated light which was transmitted through a rotary disk as the basis to detect the rotational angle of a rotatable body, and reflection types which use modulated light which was reflected at a rotary disk as the basis to detect the rotational angle of a rotatable body.
In general, a transmission type rotary encoder comprises a light emitting diode, a rotary disk which is provided with a plurality of optical tracks for converting emitted light from the light emitting diode to modulated light, a light receiving element which receives the modulated light from the rotary disk and converts it to an electrical signal, and a circuit part which processes the electrical signal from the light receiving element to calculate the rotational angle of the rotatable body. Further, each of the above-mentioned plurality of optical tracks has a structure in which parts with a light passing ability (light transmitting parts) and parts with a light interrupting ability (non-transmitting parts) are alternately arranged along an extension direction. Such a structure of an optical track is, for example, formed by etching a glass sheet on which chrome has been vapor deposited. That is, the parts of the glass sheet having the chrome layer removed by etching form the light transmitting parts of the rotary disk while the parts of the glass sheet still having the chrome layer form the non-transmitting parts of the rotary disk.
JP H11-287671A, unlike the above example, discloses forming non-transmitting parts by providing a plastic rotary disk having a light transmitting ability with a plurality of V-grooves. More specifically, in the rotary encoder of JP H11-287671A, the non-transmitting parts are formed so that the incident angle of light entering the slanted surfaces of the V-grooves is equal to or larger than a critical angle, and therefore the light which reaches the slanted surfaces of the V-grooves is completely reflected. If the above-mentioned structure of non-transmitting parts is employed, it is possible to eliminate a vapor deposition step and an etching step, and thus possible to produce the rotary disk in an inexpensive and easy manner. However, in the rotary encoder of JP H11-287671A, the emitted light from the light emitting diode is retro reflected at the V-grooves of the non-transmitting parts, and therefore the retro reflected light at the non-transmitting parts may be further reflected at the same light emitting diode. The further reflected light at the light emitting diode may travel a route which is in linear symmetrical with the route of the incident light to the rotary disk, so as to exit from light emitting diode. Usually, the rotary disk of a rotary encoder has a plurality of optical tracks which are arranged concentrically, and therefore retro reflected light at the V-groove of a certain optical track is liable to enter another optical track after being reflected at the light emitting diode. If retro reflected light at a certain optical track enters another optical track in this way, a modulated signal corresponding to the optical track may be superposed, and therefore the detection precision of the rotary encoder may be deteriorated.
In relation to this, JP2003-254785A proposes a signal processing system which is provided with a function of calculating and correcting periodic error in the detection precision of an encoder. However, the signal processing system of JP2003-254785A cannot calculate non-periodic error. Therefore, the signal processing system of JP2003-254785A can deal with deterioration of detection precision due to retro reflected light at an optical track which has a periodic pattern, but cannot deal with deterioration of detection precision due to retro reflected light of an optical track which has a non-periodic pattern. Here, a “periodic pattern” means a pattern where light transmitting parts and non-transmitting parts are alternately arranged at the same duty ratio over the entire length of an optical track. FIG. 6 is a schematic view which shows a periodic pattern of light transmitting parts L and non-transmitting parts N. Further, a “non-periodic pattern” means all patterns except for those which correspond to the above periodic pattern. FIG. 7 to FIG. 9 are schematic views which show non-periodic patterns of light transmitting parts L and non-transmitting parts N. The white parts in FIG. 6 to FIG. 9 represent the light transmitting parts L, while the hatched parts represent the non-transmitting parts N.
Further, WO2005/050141A1 proposes an encoder which controls the direction of reflected light by setting the orientation of a V-groove to 90°+α. However, if the encoder employs a light source which has a number of light reflecting surfaces such as an LED (light emitting diode) which is sealed inside a metal can package, the distance from a die pad to a light emitting point is relatively large (for example, about 2 mm), and therefore the above a has to be relatively large (see WO2005/050141A1, formula (4)). For this reason, it is actually difficult to suppress the a value to 3° or less as recommended in WO2005/050141A1. Further, WO2005/050141A1 proposes arranging a part of the rotary disk with no optical track at a location of the rotary disk which is in linear symmetrical with a non-transmitting part with respect to the optical axis of the light emitting diode. However, according to the above arrangement, the rotary disk may have larger areas which cannot be provided with optical tracks, and therefore it is not possible to make effective use of the regions illuminated by the light emitting diode.